Method of indurating ore particles



Aprxl 5, 1966 R. J. LINNEY METHOD OF INDURATING ORE PARTICLES 6Sheecs-Sheei'I 1 Filed June l0, 1964 MMURN April 5, 1966 R, J. I :NNEY

METHOD OF INDURATING ORE PARTICLES R. J. LlNNEY April 5, 1966 METHOD OFINDURATING ORE PARTICLES 6 Sheets-Sheet 3 Filed June lO, 1964 HTORNEI/5.

April 5, 1966 R. J. LlNNr-:Y 3,244,507

METHOD OF INDURTING ORE PARTICLES Filed June lO, 1964 6 Sheets-Sheet 4INVENTOR.

April 5, 1966 R. J. LINNEY 3,244,507

METHOD OF INDURATING ORE PARTICLES Filed June l0, 1964 6 Sheets-Sheet 5JPOBEErJ//v/VEK BY W 5, i966 R, LINNEY 3,244,507

METHOD OF INDURATING ORE PARTIGLES Filed June 10, 1964 6 Sheets-Sheet 6Page/P7* J L//v/VEI.

United States Patent C 3,244,507 METHUD F KNDURATING GRE PARTICLESRobert 3. Linney, Shaker Heights, Uhio, assigner to Reserve MiningCompany, Silver Bay, Minn., a corporation of Minnesota Filed June 10,1964, Ser. No. 374,119 9 Claims. (Cl. 75--3) This application is acontinuation-in-part of my copending application Serial No. 154,308,tiled November 22, 1961 (now abandoned), and also of my copendingapplication Serial No. 849,811, led October 30, 1959 (now abandoned),and which is in turn a continuationin-part of my `application Serial No.682,826, filed September9, 1957 (now abandoned).

This invention pertains to methods and apparatus for agglomeratingfinely comminuted ores and ore concentrates into pelletized form and forindurating the pellets by heat treatment for imparting thereto highhardness, strength, and resistance to fragmentation in handling orshipping.

As a convenient example, the present invention is exemplified withrespect to pelletizing concentrates of taconite ores. However, it is notlimited thereto but is applicable to the agglomeration of all types offinely comminuted ores and ore concentrates into pelletized form.

The invention nds particular application in the agglomeration andindurated pelletization of oxidic iron ores and concentrates and, moreespecially to pulverulent concentrates of the taconite ores, containingthe magnetic oxide, Fe304, which undergoes during the indurating heattreatment of the invention, an exothermic oxidation to the non-magneticor hematitic oxide, Fe203, with resultant recrystallization and graingrowth between discrete particles within each pellet which firmly bondsthem together into an integrated pelletized structure of extremely highstrength, hardness and correspondingly high resistance to fragmentationwhen subjected to the pressures and shock impacts incurred duringloading, shipping, unloading or otherwise conveying or handling.

The taconites, being low grade iron ores, cannot economically be reducedas such in the blast furnace and hence must be beneciated by crushing,grinding and magnetic separation techniques until a finely comminutedconcentrate of for example 60-65% iron content is obtained. This powderyproduct is, however, unsuitable for treatment in blast furnaces, openhearth furnaces and the like, and hence must be agglomerated intoaggregates or lumps of :suitable size and of sufcient strength for suchapplications. Various procedures to this end have heretofore beenproposed such as briquetting, sintering, pelletizing, etc. Briquettingtechniques are, however, expensive as to equipment and powerrequirements, and the briquetted product is at best quite frangible.Also certain ore and concentrate lines cannot be effectively briquettedwithout the use of binders in character and amount objectionable insubsequent reduction. Sintering is objectionable on various grounds,such as excessive fuel consumption in processing and excessive bulk ofthe product.

Pelletization as heretofore proposed or practiced has likewise provedobjectionable in various respects. Shaft kilns have been employed forthis purpose, but owing to the excessive depth and volume of the pelletbed, are diicult to control as to uniformity of feeding and temperatureof heating of the pellets, with resultant formation of sintered pelletaggregates, obstructive of gas flow and productive of overburnedaggregates. Also the green pellets entering the kilns tend to be crushedand slurried by the pressures encountered and moisture 'condensationfrom the kiln gases.

Patented Apr. 5, 1966 ICC Travelling grate processes for pelletinduration have likewise been propose-d but none prior to my inventionhas proved commercially successful insofar as I am aware, this by reasonof defects inherent therein. It has heretofore been proposed to induratea bed of green pellets fed through an ignition zone on a travellinggrate conveyor, by direct flame impingement upon the pellet bed andpenetration thereof by ames from fuel-fired burners disposed above thebed. I have found this to be highly objectionable, for one reasonbecause direct iiame impingement on the bed causes the top layer ofpellets to be fused together into a mass which is objectionable for thisreason, and also for the further reason that this fused mass has poorporosity and thus retards the flow of gas through the bed and therebylikewise prevents proper induration of the pellets at the lower bedlevels.

As applied particularly to induration of pellets agglomerated fromtaconite ore concentrates, direct flame impingement on the pellet bed isfurther objectionable for reasons as follows. My investigations haveestablished that the strongest and finest quality iron ore pelletsderived from taconite concentrates, are produced when the temperatureand oxidation conditions are such that the magnetite grains are oxidizedto hematite and sufficient grain growth occurs after this oxidation andrecrystallization to form an almost complete network of interconnectinghematite grains, as illustrated in the accompanying drawings asdiscussed below. The total strength of the pellet is related to thebridging of the hematite particles as the result of therecrystallization and grain growth process. This ideal pellet structureis obtained by controlling the actual temperature of pellet indurationat about 2300 to 2350 F. and below 2400" F.

In contrast to this, the temperature of oxidizing llames from combustionof normal commercial fuels, i.e., gas, oils or pulverized coals, isabout 2700 to 3000 F. Direct impingement of such a flame on a bed ofpellets heats some of the pellets to so high a temperature thatoxidation of the magnetite to hematite does not occur and the slagformation is so extensive that the grains of magnetite are embedded in abrittle slag matrix, also illustrated in the accompanying drawings asdiscussed below. The so-treated pellets by reason of their resultantbrittleness are easily fragmented, and thus quite inferior to pelletsindurated free from flame impingement.

Now in accordance with an important aspect of my invention, I provide atravelling grate process for progressively hardening a bed of moistgreen pellets of ore or ore concentrate nes, by subjecting the sameafter drying to a hardening heat treatment during no stage of which arethe pellets subjected to direct llame impingement from fuel burners orother heating sources. In my process the pellets are hardened by forcedflow of hot gases through the bed at a temperature such as to heat thepellets to temperature within the range of about 2300-2400 F. and belowthat productive of fusion or sintering.

A further basic objection to traveliing grate type pelletizing systemsas heretofore proposed, resides in the thermally inefficient manner inwhich they are arranged and operated with resultant excessively highfuel consumption. In such systems oxygen burned fuel, such as oil orgas, is burned throughout the length of zone required for preheating thepellet bed to indurating temperature and for maintaining the bedtemperature thereat until induration or hardening of the pellets iscompleted, whereupon the hot pellet bed is immediately cooled to lowtemperature by a forced draft of air at atmospheric temperature.Although the combustion gases after passing through the bed in theinduration zone are at a temperature of about G F., they arenevertheless immediately discharged to atmosphere as waste gases. Thisis due to the inept arrangement of compartments wherein waste gases andgases carrying sensible heat are exhausted into a common chamber,necessitating their discharge to atmosphere. Such operation is thereforeexceedingly wasteful of fuel, since only a small fraction of thechemical heat values of the fuel is usefully employed in hardening thepellets.

In accordance with a further important aspect of my invention, I largelyeliminate this defect by employing the heat from oxygen burned fuel onlyover a relatively short traverse of the pellet bed, just sufiicient topreheat a two to three inch band of the bed to indurating temerature toinitiate the hardening action therein, following which the bed is fedthrough a downwdraft reaction zone wherein the induration band is causedto progress down through the bed by air preheated to about 800 F. asderived from a subsequent updraft reaction and cooling zone throughwhich the pellet bed is fed and wherein induration -is completed and thebed is cooled by a forced draft of ambient air which is thus preheatedin passing through the hot pellet bed. In the induration of pelletscomposed of taconite ore concentrates the recuperative indurationheating obtained in this way is of course supplemented by the exothermicheat resulting from oxidation of magnetite present in the ore particlesto hematite as above explained.

I nd that as a result of the heat conservation thus obtained in myprocess, that the fuel energy consumption is only about 450,000-500,000B.t.u. per gross ton of pellets processed as compared to about700,(}-750,000 -B.t.u. per gross ton of pellets processed in accordancewith the prior art proposals above discussed. The saving on fuelconsumption is accordingly about 33 percent with my process.

I have also found that as a result of first passing the .pellets througha downdraft reaction zone and then .through an updraft zone there is, ineffect, a double .induration of the pellets and substantial improvementsin pellet quality as compared to pellets processed in accordance withprior art proposals above discussed. For example, in tests of theproduct from my process as measured against the product from prior artproposals, I found that my product had: (a) a 3.6% higher porosity whichresulted in a 6.8% faster deoxidation rate and a subsequent increase inproduction in a blast furnace; (b) an increase of 14.2% in crushingstrength and 10.5% less -28-mesh nes after a standard tumbler test whichindicates that my product is delivered to the blast furnace with agreater percentage of whole pellets and a lesser percentage of nematerial, thereby allowing greater amounts of air, with less pressuredrop, to ow through the charge in a blast furnace and hence increasingproduction; (c) a decrease of 48% in residual ferrous iron due to a morecomplete oxidation of the magnetite to hematite as a result of thesecondary induration in the updraft reaction and cooling zone whereinthe primary induration band which has progressed down to the grate inthe downdraft reaction zone is caused to reverse direction by theupdraft-of ambient air in the updraft reaction and cooling zone and thusmigrate up through the pellet bed before the sensible heat containedtherein is given up to heat the air used for recuperation. Thissecondary induration also provides for additional grain growth, therebyincreasing pellet strength.

In my process I nd that only about one-half of the preheated air yfromthe updraft reaction and cooling zone is required for primary indurationof the pellets in the downdraft reaction zone, the balance beingavailable and utilized for supplying preheated air to the combustionheating zone and also to a downdraft drying zone through which thepellet bed is fed for drying the moisture-laden, green pellets prior toheating to indurating temperature.

Having thus generally described the basic features of my invention,reference will now be had for a more de- Cil tailed description of theseand other features to the accompanying drawings, wherein:

FIG. la is a simplified, diagrammatic showing in longitudinal elevationof a preferred apparatus for practicing my invention and which alsoillustrates my novel process as to operational sequences.

FIG. lb is a temperature profile of gas temperatures at successivestages of the FIG. la sequence of operations.

FIG. 2 is a more or less diagrammatic showing in longitudinal sideelevation of a preferred form of apparatus for practicing the inventioncommercially. FIG. 3 is a transverse sectional elevation as takensubstantially at 3-3 of FIG. 2. FIG. 4 is a transverse sectional elevaastaken substantially at 4 4 of FIG. 2.

FIG. 5 is a diagrammatic showing in longitudinal side elevation of theindurating band commencing in the downdraft combustion heating zone,thence down through the pellet bed in the downwdraft reaction zone andthe reversal of direction up through the pellet bed in the updraftreaction and cooling zone, thus illustrating my novel double indurationas to operational sequence.

FIG. 6 is a side elevation of a counterweighted air seal for isolatingand maintaining positive and negative pressures between updraft anddowndraft zones as practiced in my invention.

FIGS. 7 and 8 are photomicrographic showings of the pellet structureafter induration of pellets of taconite ore concentrates as indurated inaccordance with the invention Versus induration in accordance with theprior art proposals above discussed.

Referring to FIG. 1n, there is shown a gas-permeable, travelling grate,endless conveyor 10, fed about sprocket il, which is driven for movingthe grate in the direction of the arrow X. The grate is made ofarticulated links 13, provided with oppositely disposed sidewalls, whichduring horizontal travel of the grate, form continuous sidewalls, as at14, for retention of the pellet bed, indicated at P. Moist green pelletsare continuously deposited in a uniform layer onto the grate at the feedend, as at 15, from a vibrating feed screen 16, the indurated pelletsbeing discharged from the delivery end of the grate, as at 17. The grateduring its upper path of horizontal travel, is completely hooded andcompartmentalized, as at Ztl-27, inc. Certain of the compartments areinterconnected by conduits, as at 28, 29, provided with blowers, at 30,31, 32; while other ycompartments are provided with inlet or outletconduits, as at 33, 34, 35, also provided with blowers, as at 36, 3'7,38, for purposes eX- plained below. A combustion heating chamber orfurnace 39, is provided with fuel-fed sidewall burners 40, as discussedbelow.

In the operation of the pellet indurating process of my invention asillustrated in FIG. la, the bed P of the moist, green pellets as laiddown at 1S at the feed end of the travelling grate 10, is fed thencesuccessively through t-he various treating zones indicated by thelegends at the top of the drawing and comprising, updraft drying,downdraft drying, combustion heating, downdraft reaction and updraftreaction and cooling zones.

The only source of fuel-fired heat supplied to the system is by way ofthe fuel-tired burners 40 mounted in the sidewalls of the hooded chamber39 comprising a combustion chamber or combustion zone. The hot gasesfrom these burners are caused to flow in downdraft through the pelletbed and grate by the suction blower 37 in conduit 34 which connects tochamber 2.4 beneath the furnace chamber 39. Since these gases aredeficient in oxygen due to combustion of fuel in the burners 40, theyare discharged as waste gases to atmosphere via conduit 34 depleted oftheir sensible heat.

These hot gases in passing through the pellet bed from chamber 39 to 24,heat up the top two or three inches of the bed of pellets to induratingtemperature in the manner illustrated at A, FIG. 5. As the bed ofpellets,

is conveyed thence across the downdraft reaction zone 22, 23, this 'bandof induration which started on top of the bed progresses down throughthe bed as shown at B, FIG. 5, until the two to four inch band reachesthe grate 41, as at C, just before the pellets are conveyed into theupdraft reaction and cooling zone 21, 20.

The recuperation is initiated in chamber 20 wherein the hot pellets aresubjected to an updraft of cool, ambient air forced into the lowercooling chamber 20 via conduit 33 and forced draft blower 36, FIG. 1a.This updraft of cool air impinges first on the conveyor grate bars, asat 41, FIG. 5, thus to cool the same to prevent warpage and distortion,and the heat from which is caused to flow up through the pellet bedalong with cool air, as at D, FIG. 5, thereby reversing the direction ofthe indurat-ing Lband causing it to migrate up through the bed and atthe same time cooling the bed beneath the indurating band and alsoIheating the air iowing into upper chamber 21. As shown by the gastemperature diagram of FIG. 1b, the thus heated air enters chamber 21 ata temperature of about 800 F., while meantime cornpleting induration andcooling the grate bars and pellet bed down to temperature of about 200F. for discharge.

The upper reaction and cooling chamber 21 opens into the upper downdraftreaction chamber 23, as indicated by the arrow, and a portion,preferably about one-half of the 800 F. air flowing into chamber 21, iscaused to iiow into the upper downdraft reaction zone chamber 23 andthence in downdraft through the pellet bed in the downdraft reactionzone by virtue of the suction blower 31 in conduit 29, FIG. 1a,connected to the lower cham- -ber 22 of the downdraft reaction zone.

The thus preheated air from chamber 21 in passing in downdraft throughthe pellet bed completes primary induration of t-he pellets initiated inthe combustion heating zone. As shown by FIG. 1b, the downdraft airafter passing through the pellet in the downdraft reaction zone andlafter tempering in the manifold 22, has a temperature of about 800 F.It is fed via conduit 29 and vblower 31, thence through a second blower32 into the lower drying zone chamber 26, and thence in updraft throughthe travelling grate and pellet bed into the upper drying zone chamber27, from whence moisture-laden, it is discharged to atmosphere viaconduit 35 containing the suction blower 3S, at temperature of about 200F., as shown by reference to FIG. 1b.

The hot dry air entering the lower dry-ing zone chamber 26 at about 800F., in passing in updraft through the pellet bed, is thus most effectivein drying t-he pellets in the lower portion of the bed thereby to greenharden and strengthen the same, and it also provides a supporting columnof uprising air to prevent crushing by the upper layer of pellets. Alsoin this way such moisture condensation as occurs on the cold pelletswill do so only on the upper layer of pellets vhaving minimum loadthereon, thereby to prevent injury by crushing.

My investigations have shown that if the hot air is first passed indowndraft through the cold, entering bed of pellets, that although thepellets in the upper layer of t-he -bed will be dried and greenhardened, those in the lower layer will slurry and crush due to moisturecondensation thereon from the now moisture-laden, heated air, which inpassing through the upper pellet layer has picked up considerablemoisture, and that the wet slurried pellets in the lower part of the bedwill crush due to being subjected to the pressure of the downdraftcolumn of air and the weight of the pellet bed.

Reverting to the discharge end 17 of the grate, as above stated, onlyabout one-half of the heated air flowing into the upper updraft reactionand cooling zone chamber 21, is caused to flow into the upper downdraftreaction zone chamber 23. The remainder is drawn olf from chamber 21over conduit 23 through the suction blower 30, and conveyed thence tothe downdraft drying and combustion heating zone chambers 25 and 39. Theportion entering the downdraft drying zone chamber 25, passes indowndraft through the pellet :bed to further dry and green harden thepellets by removing residual moisture therefrom, with particularreference to the residual moisture contained in the upper pellet layer.The portion entering the combustion heating zone chamber 39, serves toprovide preheated air to the burners 40 and also to the combustionheating zone chamber 39, and thus reduces to that extent the amount ofthermal energy to be supplied by fuel consumed by the burners forheating the upper part of the pellet .bed up to the induratingtemperature of about 2300-2400 F.

As above stated and as described more in detail below with reference toFIG. 3, the burners 40 are so mounted, flame adjusted and positionedabove the pellet bed, that there occurs no direct impingement of flamefrom the burners on the pellet bed. Only hot gases heated by the burnerscontact the bed being drawn therethrough as explained by the suctionblower 37 in conduit 34 connected to the 4lower combustion Zone chamber24. The temperature of these gases is so adjusted as to heat the upperlayer of the pellet bed to indurating temperature of about 2300-2400 F.and thus to initiate the indurating reaction as the bed passes into thedowndraft reaction zone, wherein the primary induration reaction iscompleted in the manner above explained. That is to say, as the pelletbed traverses the downdraft reaction zone, a wave of heat at induratingtemperature is caused to pass downwardly through the bed due to theforced iiow downdraft of preheated air from chamber 21 passing throughthe bed while the exothermic reaction of the magnetite to hematiteoxidation occurring therein aids in holding the pelletizing temperatureat optimum.

As shown by FIG. 1b the gases from the lower chamber 24 of thecombustion heating zone exit to atmosphere at only about 400 F. This isin marked contrast to the prior art systems above discussed whereinthese gases exit at about 1000" F. This marked difference in temperatureof exit waste gases in my process versus the prior art, results from theheat conservation obtained by recuperation in my process. Thisrecuperation takes dry air at about 800 F. containing an oxygen level ofof ambient from chamber 21 as preheated by updraft reaction and coolingof the hot indurated pellets and employs the heat values thereof forprimary induration of pellets in the downdraft reaction zone and thencefor updraft drying in the updraft drying zone, the air from which isexhausted to atmosphere at the low temperature of about 200 F. Inaddition, blower 30 transports part of the 800 F. air from the updraftreaction and cooling chamber 21 to the combustion heating chamber 39, tothe burners 40, and to the downdraft drying zone 25, these gases passingthence through the pellet bed to atmosphere at 400 F. The prior art isdevoid of any such teaching or suggestion insofar as I am aware.

Referring now to the commercial embodiment of my invention as shown inFIGS. 2-6, inc., equivalent elements have been similarly designated asin FIG. 1a from which the identity of equivalent components will be apparent. However, there are some structural modifications in the FIGS.2-6, inc., embodiment which will become apparent by comparison with thesimplified showing of FIG. la. Thus, referring to FIGS. 2-6, inc., aseries of funnel-like wind boxes, as at 50, 51, are disposed seriatimsubstantially the length of the traveling grate 10, and are positionedwith the ared openings just beneath the grate, as at 52, 53. A series ofmanifolds 20, 22, 24, 26, corresponding in function to the like numberedlower chambers of FIG. 1a, are disposed seriatim below the wind boxes asshown, and from each wind box a conduit extends downwardly to themanifold positioned beneath the same. Thus, conduits 54, 55, extend fromwind boxes 50, 51, down to manifold 26, positioned beneath them, and soon for the remaining wind boxes. The manifolds, accordingly function tosegregate the wind boxes into groups corresponding to the lower likenumbered lower drying, combustion heating, downdraft reaction andupdraft reaction and cooling compartments of FIG. la.

Above grate 10, are disposed compartmentalized hoods 27, 25, 39, 23 and21, corresponding to the like numbered upper hood compartments orchambers of FIG. la, for providing the upper drying, combustion heating,downdraft reaction and updraft reaction and cooling compartments.Likewise, in FIGS. 2-6, inc., the motor driven blowers and conduitscontaining them are arranged and connected the saine as the likenumbered components of FIG. la and for performing the same functions,respectively. Referring to FIGS. 3 and 4, oppositely disposed sidepanels 56 and 57, extend down from the upper hoods to hoppers 7S, inorder to enclose the grate 1t) substantially throughout its length,thereby making the entire enclosure a pressure balancing plenum chambersimilar to that described in U.S. Patent 3,088,723. This tunnel-likehousing encloses the grate from the upper hoods 21, 23, 39, 25 and 27 tobelow the return strand of the grate by walls 56 and 57 as shown inFIGS. 3 and 4. The entire tunnel-like structure is under a slightnegative pressure and acts as a vacuum cleaner to contain essentiallyall of the air-borne dust in the system. The tunnel also acts as apressure equalizing plenum chamber to contain the small surges ofpositive or .negative pressure in the air flow system of the machinerather than allowing them to dissipate to atmosphere.

In addition to the above described dust collection system, dustcollecting means are provided both at the feed end and discharge end ofgrate 1t). Referring to FIG. 2, at the feed end, wind box 95 isconnected to updraft drying chamber 27 by conduit 96. Wind box 95 isunder slight negative pressure, being exhausted by blower 33, and picksup air-borne dust from the slight blow-by through air seal 94, FIGS. 2and 6.

Referring to FIG. 6, the air seal mechanism shown generally at 94,comprises an air sealing plate member 100, pivotally mounted as at 101,to a supporting structure 102 of the wind box assembly, the air sealingplate member having integral therewith a counterweight 193, ofsufficient magnitude to maintain the sealing plate and counterweightassembly in the position shown by the dotted lines 100g, 103a, wherebythe outer edge of the sealing plate rides along the underside of thepallets or articulated links 13 of the travelling grate conveyor 10,thus to minimize blow-by of air between the sealing plate and the grate.

Reverting to FIG. 2, the discharge end of grate is enclosed by dustcollecting hood 97, whereby dust is exhausted to dust collectors, notshown, via conduit 93 and is eventually returned to the system. Theblow-by in the terminal wind box is controlled to a minimum by air seal91 similar to that shown in FIG. 6. Two additional and similar air seals92 and 93, FIG. 2, effectively prevent the short-circuiting of airbetween the pressurized wind box on one side of the seal and the suctionwind box on the other side of the seal.

As further shown in FIG. 2, damper controls are provided in the variousconduits, `as at 60, 61, 62, for controlling the dow of airtherethrough. There are also tempering air dampers, as at 67, 68 and 69,to permit ambient air to be drawn into the respective ducts or chambersto control the temperature of the gases passing through blowers fortheir protection against excessive temperatures. Conduit 2S is branchedas at 63, 64, for directing desired fractions of the hot updraft airfrom chamber 211 into the combustion heating and downdraft dryingchambers 3-9, 25, on the one hand, and as preheated air supply to thefuel burners 40, on the other. The lower manifold 2i) in the updraftreaction and cooling zone is provided with removable sections, as at 65,66, `for increasing or decreasing the effective length of S thismanifold, and thereby correspondingly varying the eective length of theupdraft reaction and cooling zone.

Referring to FIG. 3 the burners 4t) are fitted as above stated intoports, as at 70, extending through sidewalls, as at 71, 72, of thecombustion furnace chamber 39. By reason of this mounting the burnersdirect their flames horizontally into the combustion heating chamber 39at a level well above the pellet bed P of the travelling grate 10, inthe manner illustrated at '73. Thus, there occurs no direct impingementof the flames 73 from the burners di) onto the lpellet bed P. Thella-mes are additionally adjusted to assure this by controlling therates of air and fuel supply to the burners over the air lines, as at74, and fuel lines, as at 75. The air supplied to the air lines 74 ispreheated air supplied over conduits 28 and 64, FIG. 2. The air fed intothe combustion heating chamber through the inlet conduits 76 is thepreheated air supplied thereto over conduits 28 and 63, FIG. 2.

The preferred pellet size is about 11/2 to 3@ inch. The screen 16 whichfeeds the green pellet agglomerates into the yfeed end of the travellinggrate is of a mesh to retain pellets of 1A inch but to reject smallersizes. The grate bars of the travelling grate are likewise spaced toretain pellets of about 1A inch and over but to pass smaller sizes.Referring to FIGS. 3 and 4, pellets fragmented during lfeed over thetravelling grate or those that fall through openings caused by brokengrate bars, fall into hoppers, as at 77, 78, at the base of the wind boxconduits, and are removed on travelling belts, as at 79, 80.

As was pointed out above, taconite ore pellets as indurated at aboutZ300-2400" F. in the absence of direct ame impingement on the pellet bedproduce the strongest and finest quality iron pellets when thetemperature and oxidation conditions are such that the magnetite grainsare oxidized to hematite and sufcient grain growth occurs after thisoxidation and recrystallization to form an almost complete network ofinterconnecting hematite grains. The microstructure of pellets as thusproduced in accordance ywith the present invention is shown in FIG. 7 ofthe drawings. As there shown the microstructure consists of a 'Wellbridged network of hematite grains comprising the white areas. It willbe noted that the slag content comprising the gray areas is generallyconned to coalesced patches within the individual grains or as longerisolated particles. The voids are shown in black.

It was further pointed out in contrast to this desired, induratedmicrostructure, that if the pellets are indurated by direct ilameimpingement on the pellet bed, this heats at least the surface layer ofpellets to so high a temperature that oxidation of the magnetite tohematite does not occur, as a result of which the slag formation is soextensive that the grains of magnetite are embedded in a brittle slagmatrix. The microstructure obtained in this way is illustrated in FIG. 8of the drawings. In this View the magnetite grains are shown in gray,with a complete slag network, shown in darker gray, surrounding the.magnetite grains, the interstices or voids being again shown in black.

What is claimed is:

1. The method of hardening and strengthening moist, green pellets ofore-bearing particles, which comprises: continuously depositing saidpellets in a substantially uniform bed on a gas-permeable, travellinggrate conveyor and feeding successively through drying, combustionheating, indurating reaction and cooling and reaction zones, generatingheated gases in said combustion heating zone and passing through saidbed thereat to heat said pellets, passing cool air through said bed insaid cooling and reaction zone to cool said bed and heat said air,passing a substantial portion of said heated air through said bed insaid indurating reaction zone and thence through said bed in said dryingzone, passing a substantial portion of the heated air from said coolingand reaction zone to said combustion heating zone to supplement the heatof the gases passed through said bed thereat, and adjusting thetemperature and the rate of ow of the heated gases passed through saidbed in said cornbustion heating zone and the rates of flow of said gasespassed through said bed in said cooling and reaction, induratingreaction and drying zones, to dry said pellets in said drying zone, toinitiate induration of said pellets in said combustion heating zone, tocontinue the induration of said pellets in said indurating reaction zoneand to complete induration of said pellets and cool said pellets in saidcooling and reaction zone.

2. The method of hardening and strengthening moist, green pellets ofore-bearing particles, which comprises: continuously depositing saidpellets in a substantially uniform bed on a gas-permeable, travellinggrate conveyor and feeding successively through drying, combustionheating, downdraft reaction, and updraft reaction and cooling zones,generating heated gases in said combustion heating zone and passing indowndraft through said bed thereat to heat said pellets, passing coolair in updraft through said bed in said updraft reaction and coolingzone to cool said grate and bed and heat said air, passing a substantialportion of said heated air in downdraft through said bed in saiddowndraft reaction zone and thence in updraft through said bed in saiddrying zone, passing a substantial portion of the heated air from saidupdraift reaction and cooling zone to said combustion heating zone tosupplement the heat of the gases passed through said bed thereat, andadjusting the temperature and the rate of ow of the heated gases lpassedthrough said bed in said combustion heating zone and the rates of flowof said gases passed through said -bed in said updraft reaction andcooling z-one, and said downdraft reaction and updraft drying zones, todry said pellets in said drying zone, to initiate hardening of saidpellets in said combustion heating zone, to continue the hardening ofsaid pellets in said downdraft reaction zone and to complete hardeningof said pellets and cool said pellets in said updraft reaction andcooling zone.

3. The method of hardening and strengthening moist, green pellets ofore-bearing particles, which comprises: continuously depositing saidpellets in a substantially uniform bed on a gas-permeable, travellinggrate conveyor and feeding successively through drying, combustionheating, downdraft reaction and updraft reaction and cooling zones,generating heated gases in said combustion heating zone and passing indowndrat through said bed thereat to heat said pellets, passing cool airin updraft through said bed in said updraft reaction and cooling zone tocool said grate and bed and heat said air, passing a substantial portionof said heated air in downdraft through said bed in said downdraftreaction zone and thence in updraft through an entrance portion of saidbed in said drying zone, passing a substantial portion of the heated airfrom said updraft reaction and cooling zone in downdraft through aterminal portion of said drying zone, and another substantial portion tosaid combustion heating zone to supplement the heat of the gases passedthrough said bed thereat, and adjusting the temperature and the rate ofow of the heated gases passed through said bed in said combustionheating zone and the rates of ow of said gases passed through said bedin said updraft reaction and cooling zone, and said downdraft reac* tionand drying zones, to dry said pellets in said drying zone, to initiatehardening of said pellets in said combustion heating zone, to continuethe hardening of said pellets in said downdraft reaction zone and tocomplete hardening of said pellets and to cool said pellets in saidupdraft reaction and cooling zone.

4. The method of hardening and strengthening moist, green pellets ofore-bearing particles, which comprises: continuously depositing saidpellets in a substantially uniform bed on a gas-permeable, travellinggrate conveyor and feeding successively through drying, combustionheating, indurating reaction and cooling and reaction zones, generatingheated gases by burning fuel in said combustion heating zone and passingsaid heated gases through said bed thereat to heat said pellets Withoutflame impingement from said burning fuel on said bed, passing cool airthrough said bed in said cooling and reaction zone to cool said bed andheat said air, passing a substantial portion of said heated air throughsaid bed in said indurating reaction zone and thence through said bed insaid drying zone, passing a substantial portion of the heated air fromsaid cooling and reaction zone to said combustion heating zone tosupplement the heat of the gases passed through said bed thereat, andadjusting the temperature and the rate of flow of the heated gasespassed through said bed in said combustion heating zone and the rates oftlow of said gases passed through said bed in said cooling and reaction,indurating reaction and drying zones, to dry said pellets in said dryingzone, to initiate hardening of said pellets in said combustion heatingzone, to continue the hardening of said pellets in said induratingreaction zone without fusion or sintering of said pellets, and tocomplete induration of said pellets and to cool said pellets in saidcooling and reaction zone.

S. The method of hardening and strengthening moist, green pellets ofmagnetite-containing particles, which comprises: continuously depositingsaid pellets in a substantially uniform bed on a gas-permeable,travelling grate conveyor and feeding successively through drying,combustion heating, indurating reaction and cooling and reaction zones,generating heated oxidizing gases in said combustion heating zone andpassing through said bed thereat to heat said pellets, passing cool airthrough said bed in said cooling and reaction zone to cool said bed andheat said air, passing a substantial portion of said heated air throughsaid bed in said indurating reaction zone and thence through said bed insaid drying zone, passing a substantial portion of the heated air fromsaid cooling and reaction zone to said combustion heating zone tosupplement the heat of the gases passed through said bed thereat, andadjusting the temperature and the rate of oW of the heated gases passedthrough said bed in said combustion heating zone and the rates of tow ofsaid gases passed through said bed in said cooling and reaction,indurating reaction and drying zones, to dry said pellets in said dryingzone, to initiate hardening of said pellets in said combustion heatingzone and to complete the hardening thereof in said indurating reactionand said cooling and reaction zones by exothermic oxidation of saidmagnetite particles to hematite and by integrating recrystallization andgrain growth, and also to cool said pellets in said cooling and reactionzone.

6. The method of hardening and strengthening moist, green pellets ofmagnetitecontaining particles, which comprises: continuously depositingsaid pellets in a substantially uniform bed on a gas-permeable,travelling grate conveyor and feeding successively through drying,combustion heating, indurating reaction and cooling and reaction zones,generating heated oxidizing gases by burning fuel in said combustionheating zone and passing said heated gases through said bed thereat toheat said pellets but Without ame impingcment from said burning fuel onsaid bed, passing cool air through said bed in said cooling and reactionzone to cool said bed and heat said air, passing a substantial portionof said heated air through said bed in said indurating reaction zone andthence through said bed in said drying zone, passing a substantialportion of the heated air from said cooling and reaction zone to saidcombustion heating zone to supplement the heat of the gases passedthrough said bed thereat, and adjusting the temperature and the rate offlow of the heated gases passed through said bed in said combustionheating zone and the rates of flow of said gases passed through said bedin said cooling and reaction, indurating reaction and drying zones, todry said pellets in said drying zone, to

initiate hardening of said pellets in said combustion heating zone andto complete the hardening of said pellets in said indurating reactionand said cooling and reaction zones by exothermic oxidation of saidmagnetite particles to hematite and by integrating recrystallization andgrain growth, and also to cool said vpellets in said cooling andreaction zone.

7. The method of hardening and strengthening moist, green pelletscontaining hematite particles and particles of a solid carbonaceousfuel, which method comprises: continuously depositing said pellets in asubstantially uniform bed on a gas-permeable, travelling grate conveyorand feeding successively through drying, combustion heating, induratingreaction and cooling and reaction zones, generating heated gases in saidcombustion heating zone and passing through said bed thereat to heatsaid pellets, passing cool air through said bed in said cooling andreaction zone to cool said bed and heat said air, passing a substantialportion of said heated air through. said bed in said indurating reactionzone and thence through said bed in said drying zone, passing asubstantial portion of the heated air from said cooling and reactionzone to said combustion heating zone to supplement the heat of the gasespassed through said bed thereat, and adjusting the temperature and therate of flow of the heated gases passed through said bed in saidcombustion heating zone and the rates of iiow of said gases passedthrough said bed in said cooling and reaction, indurating reaction anddrying zones, to dry said pellets in said drying zone, to initiatehardening of said pellets in said combustion heating zone, and tocomplete the hardening of said pellets in said indurating reaction andsaid cooling and reaction zones and to also cool said pellets in saidcooling and reaction zones.

8. The method of hardening and strengthening moist, green pellets ofore-bearing particles, which comprises: continuously depositing saidpellets in a substantially uniform bed on a gas-permeable, travellinggrate conveyor and feeding successively through drying, combustionheating, downdraft reaction, and updraft reaction and cooling zones,generating heated gases in said combustion heating zone and passing indowndraft through said bed thereat to heat said pellets in an upper bandof said bed to indurating temperature, passing cool air in updraftthrough said bed in said updraft reaction and cooling zone to cool saidgrate and bed and heat said air, and passing'a substantial portion ofsaid heated air in downdraft through said bed in said downdraft reactionzone, thereby to propagate said indurating temperature band downwardlythrough said pellet bed in said downdratt reaction zene and thenceupwardly through said bed in said updraft cooling and reaction zone,passing the gases fed through said downdraft reaction zone in updraftthrough said bed in said drying zone, and passing a substantial portionof the heated air from said updraft reaction and cooling zone to saidcombustion heating zone to supplement the heat of the gases passedthrough said bed thereat.

9. The method of hardening and strengthening moist, green pellets ofore-bearing particles, which comprises: continuously depositing saidpellets in a substantially uniform bed on a gas-permeable, travellinggrate conveyor and feeding successively through drying, combustionheating, downdraft reaction and updraft reaction and cooling zones,generating heated gases in said combustion heating zone and passing indowndraft through said bed thereat to heat said pellets in an upper bandthereof to indurating temperature, passing cool air in updraft throughsaid bed in said updraft reaction and cooling zone to cool said grateand bed and Vheat said air and passing a substantial portion of saidheated air in downdraft through said bed in said downdraft reaction zonethereby to propagate said indurating temperature band downwardly throughsaid bed in said downdraft reaction zone and thence upwardly throughsaid bed in said updraft cooling and reaction Zone, passing the gases'fed through said downdraft reaction zone in updraft through an entranceportion of said bed in said drying zone, passing a substantial portionof the heated air from said updraft reaction and cooling zone indowndraft through a terminal portion of said drying zone, and anothersubstantial portion to said combustion heating zone to supplement theheat of the gases passed through said bed thereat.

References Cited bythe Examiner UNlTED STATES PATENTS 2,506,569 5/1950Agnew 266-21 2,750,272 6/1956 Lellep 75-3 2,750,274 6/1956 Lellep 75--32,768,890 10/1956 Cover 755 2,987,307 6/1961 Homan 266-21 3,003,86310/1961 Meyer et al. 75-5 BENJAMN HENKIN, Primary Examiner.

1. THE METHOD OF HARDENING AND STRENGTHENING MOIST, GREEN PELLETS OFORE-BEARING PARTICLES, WHICH COMPRISES; CONTINUOUSLY DEPOSITING SAIDPELLETS IN A SUBSTANTIALLY UNIFORM BED ON A GAS-PERMEABLE, TRAVELINGGRATE CONVEYOR AND FEEDING SUCCESSIVELY THROUGH DRYING, COMBUSTIONHEATING, INDURATING REACTION AND COOLING AND REACTION ZONES, GENERATINGHEATED GASES IN SAID COMBUSTION HEATING ZONE AND PASSING THROUGH SAIDBED THREAT TO HEAT SAID PELLETS, PASSING COOL AIR THROUGH SAID BED INSAID COOLING AND REACTION ZONE TO COOL SAID BED AND HEAT SAID AIR,PASSING A SUBSTANTIAL PORTION OF SAID HEATED AIR THROUGH SAID BED INSAID INDURATING REACTION ZONE AND THENCE THROUGH SAID BED IN SAID DRYINGZONE, PASSING A SUBSTANTIAL PORTION OF THE HEATED AIR FROM SAID COOLINGAND REACTION ZONE TO SAID COMBUSTION HEATING ZONE TO SUPPLEMENT THE HEATOF THE GASES PASSED THROUGH SAID BED THEREAT, AND ADJUSTING THETEMPERATURE AND THE RATE OF FLOW OF THE HEATED GASES PASSED THROUGH SAIDBED IN SAID COMBUSTION HEATING ZONE AND THE RATES OF FLOW OF SAID GASESPASSED THROUGH SAID BED IN SAID COOLING AND REACTION, INDURATINGREACTION AND DRYING ZONES, TO DRY SAID PELLETS IN SAID DRYING ZONE, TOINITIATE INDURATION OF SAID PELLETS IN SAID COMBUSTION HEATING ZONE, TOCONTINUE THE INDURATION OF SAID PELLETS IN SAID INDURATING REACITON ZONEAND TO COMPLETE INDURATION OF SAID PELLETS AND COOL SAID PELLETS IN SAIDCOOLING AND REACTION ZONE.